Abstract
Abstract. Volatile inorganic and size-resolved particulate Cl- and Br-species were measured in near-surface air over a broad range of conditions within four distinct regimes (European – EURO, North African – N-AFR, the Intertropical Convergence Zone – ITCZ, and South Atlantic – S-ATL) along a latitudinal gradient from 51° N to 18° S through the eastern Atlantic Ocean. Median dry-deposition fluxes of sea salt, oxidized N, and oxidized non-sea-salt S varied by factors of 25, 17, and 9, respectively, among the regimes. Sea-salt production was the primary source for inorganic Cl and Br. Acidification and dechlorination of sea salt primarily by HNO3 sustained HCl mixing ratios ranging from medians of 82 (ITCZ) to 682 (EURO) pmol mol−1. Median aerosol pHs inferred from HCl phase partitioning with super-μm size fractions ranged from ~3.0 for EURO to ~4.5 for ITCZ. Because SO2 solubility over this pH range was low, S(IV) oxidation by hypohalous acids was unimportant under most conditions. Simulations with a detailed multiphase box model indicated that BrCl photolysis and ClO + NO were the major sources for atomic Cl in all regimes. Simulated midday concentrations of Cl atoms ranged from 2.1×104 to 7.8×104 cm−3 in the ITCZ and N-AFR regimes, respectively. Measured particulate Br− (median enrichment factor = 0.25) was greater and volatile inorganic Br less than simulated values, suggesting that the halogen activation mechanism in the model overestimated Br-radical production and processing. Reaction with atomic Br was an important sink for modeled O3 (5% in EURO to 46% in N-AFR). Formation of halogen nitrates accelerated the oxidation of NOx (NO + NO2) primarily via hydrolysis reactions involving S aerosol. Relative to simulations with no halogens, lower NOx coupled with direct reactions involving halogens yielded lower steady state mixing ratios of OH (20% to 54%) and O3 (22% to 62%) and lower midday ratios of OH:HO2 (3% to 32%) in all regimes.
Highlights
Chemical reactions involving inorganic halogens significantly influence the composition of the Earth’s atmosphere
marine boundary layer (MBL) depth was estimated from 1) profiles of virtual potential temperature derived from rawinsondes launched from the ship at 12:00 UTC on most days and at other times on some days, 2) the lowest cloud base measured by a ceilometer operated by the ship’s crew, and 3) either the first significant drop in back scatter or the lowest cloud layer detected with a lidar operated during the cruise (Immler and Schrems, 2006)
Because spatial variability in the speciation and relative importance of different sources along the cruise track could not be evaluated explicitly, the model was parameterized by adjusting the HNO3:NO2 ratio in the oxidized N source term for each regime such that corresponding simulated steady-state mixing ratios of NOx in the model runs that included transformations involving halogens fell within the range of available measurements in the eastern Atlantic MBL (e.g., Carsey et al, 1997; Leser et al, 2003; Lawler et al, 2009; Lee et al, 2009)
Summary
Chemical reactions involving inorganic halogens significantly influence the composition of the Earth’s atmosphere. Despite the potential importance of reactive halogen chemistry, comprehensive multiphase observations over a broad range of tropospheric conditions, and over the open ocean, are scarce thereby constraining our ability to reliably predict associated influences on O3 cycling, oxidation processes, aerosol evolution, and radiative transfer regionally and globally.
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